FIG. 6 shows one example of a conventional synthetic resin cap, in which a synthetic resin cap 41 comprises a cap body 4 composed of a top plate 2 and a cylindrical section 3 extending downward from its periphery.
The cylindrical section 3 is divided by a horizontal score 6 into a main section 8 and a TE ring section 9, which is connected to the bottom edge of the main section 8 by bridges 7.
A circular inner seal projection 12 fits into a container opening, and protrudes from the inner surface of the top plate 2.
A threaded section 40 that engages with a thread of the container opening is formed in the inner peripheral surface of the main section 8. The circumferential direction forming angle of the threaded section 40, i.e. the angle from the top section 40a to the bottom section 40b, is generally set at approximately 540° (approximately 1.5 times the circumference).
In the cap 41, the threaded section 40 is divided into a plurality of divided threaded sections 42 and 43 in the lengthwise direction.
In the cap 41, divided sections 44 divide the threaded section 40 into the divided threaded sections 42 and 43, and are provided only at sections where the threaded section 40 is formed in two steps. That is, the threaded section 40 comprises one long divided threaded section 42 that is formed in one step, and a plurality of short divided threaded sections 43 that are formed in two steps.
In manufacturing the cap 41, synthetic resin material is generally pressure-molded by using molds for outer surface and inner surface, the molded cap 41 being removed from the mold for outer surface, and the mold for inner surface being peeled away from the cap 41.
In the conventional cap 41, since the divided threaded sections 42 and 43 have different lengths and numbers of formation, the strength of the threaded section 40 at the section where the divided threaded section 42 is formed differs from its strength at the sections where the divided threaded sections 43 are formed.
As a consequence, when a container which the cap 41 is attached to has high internal pressure (e.g. when the cap 41 has been attached to a container filled with a fizzy beverage, or when, after removing the cap 41 and then reattaching it, the contained liquid has fermented, and other such cases), the weaker divided threaded section 42 of the threaded section 40 will not mesh adequately with the thread of the container opening, making the cap 41 unable to be attached properly and reducing its hermeticity.
Furthermore, since the strength of the threaded section 40 of the cap 41 inclines in the circumferential direction, at the time of molding, and particularly at the time of peeling away the mold for inner surface from the cap 41, the cap 41 may become tilted with respect to the mold for inner surface, applying an excessive force against part of the threaded section 40 and causing this part to deform. When such deformation has occurred, the cap 41 cannot be attached properly and its hermeticity decreases.